Why more undocumented migrants are reaching America

DONALD TRUMP promised to resort to untested measures to keep Mexican migrants from crossing America’s southern border. The promise contained at least two nagging flaws. The first is an outdated view. Migration of Mexicans is down by 90% from its peak in 2000; now most border-hoppers come from the “Northern Triangle” of Guatemala, Honduras and El Salvador. The second error was to rile Mexico with insults and threats when America relies on its goodwill to police its own southern border, which migrants must first cross before continuing on to America.

In February the number of migrants stopped while trying to enter America from Mexico—a proxy for overall illegal migration levels—rose to 76,000. That is the highest number for any month in a decade. The increase consisted almost entirely of Central Americans, not Mexicans. Meanwhile, Mexican authorities have been deporting less than half as many Central Americans as usual since Andrés Manuel López Obrador, a left-winger, took office in December. Mexico deported one migrant for every four that were apprehended in America in the year before he took office. Now the ratio closer to one to ten.

That is no coincidence. Mr López Obrador’s team vows to depart from the “mass deportations” of migrants that Mexico has carried out since 2014 at America’s behest. In January, confronted...

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